1. FIELD OF THE INVENTION
This invention relates generally to scanning electron microscopes, and more particularly to a scanning electron microscope suitable for stereoscopic observation of a specimen.
2. DESCRIPTION OF THE RELATED ART
In a scanning electron microscope, an electron beam emitted from an electron gun is focused on a specimen by an objective lens. The specimen is two-dimensionally scanned by the focused electron beam, and, as a result of scanning, an information signal characterizing the specimen is generated from the specimen. Any of secondary electrons, reflected electrons, absorbed electrons, X-rays and cathode luminescence may be used as sources for providing such an information signal. The information signal is detected and applied to a cathode-ray tube for the purpose of brightness modulation. On the other hand, two-dimensional scanning of the display screen of the cathode-ray tube by cathode rays (an electron beam) is effected in synchronism with the two-dimensional scanning of the specimen by the focused electron beam. Therefore, an image of the scanned region of the specimen based on the information signal generated from the specimen, that is, a scanned specimen image is displayed on the screen of the cathode-ray tube.
Also, according to the scanning electron microscope, stereoscopic observation of scanned specimen images is frequently desired. It is necessary to direct the electron beam toward and onto the specimen at a selected angle of incidence. Two methods are known for directing the electron beam toward and onto the specimen in an angular relation. According to one of these methods, the direction of the electron beam is fixed, while the specimen is mechanically inclined. On the other hand, according to the other method, the specimen is fixed, while the electron beam is deflected. However, in the case of the former method, a large-sized device for inclining the specimen is required when the specimen has a large size, and the distance between the objective lens and the specimen, that is, the working distance becomes inevitably large when the specimen is to be inclined through a large angle. Thus, when the size of the specimen is large, the latter method is preferably employed.
According to JP-A-58-147948 which discloses a scanning electron microscope and is already known, a parallel electron beam is directed toward a position on the principal plane of the microscope's objective lens, which position is outside the axis of the objective lens. (This position will be referred to hereinafter as an off-axis position). As a result, the parallel electron beam is focused on a desired spot of a specimen by the objective lens and incident on the desired spot at a selected angle of incidence. In this case, in order to attain two-dimensional scanning of the specimen by the electron beam focused to obtain a scanned specimen image, it is proposed that the deflection fulcrum for the electron beam used for scanning the specimen is to be located (1) at a position where the deflection fulcrum coincides with the deflection fulcrum of the parallel electron beam deflected to be directed toward the off-axis position on the principle plane of the objective lens; or (2) at a position on the principal plane of the objective lens; or (3) at a position between the objective lens and the specimen.
However, in the case of the proposal (1), the parallel electron beam moves on the principal plane of the objective lens during scanning the specimen, resulting in an increased distortion of the scanned specimen image. In order to minimize this image distortion, it is necessary to increase the aperture of the objective lens. In the case of the proposal (2), the beam deflecting means for specimen scanning purpose must be disposed inside the objective lens. Therefore, it is necessary to correspondingly increase the aperture of the objective lens. In the case of the proposal (3), the beam deflecting means for specimen scanning purposes must be disposed between the specimen and the objective lens. Therefore, it is necessary to correspondingly increase the working distance. When the angle of incidence of the electron beam on the specimen is supposed to be fixed, and the working distance is increased in such a case, the aperture of the objective lens must be correspondingly increased.
The fact that the objective lens has a large aperture means that the objective lens has a long focal distance. Thus, the coefficient of chromatic aberration Cc increases, and the resolution is greatly lowered. This is because the resolution is degraded in proportion to the half power (Cc.sup.1/2) of the coefficient of chromatic aberration Cc. The sam also applies when the working distance increases. This is because the focal distance of the objective lens increases.
In the scanning electron microscope, it is desirable to change the strength of electron beam current irradiating the specimen or to change the beam reduction rate of the electron-beam optical system, and, of this purpose, it is customary to focus the electrons beam at least once at a position between the electron gun and the objective lens. Therefore, in order to cause incidence of the parallel electron beam on the objective lens, it becomes necessary to additionally provide a lens which converts the focused electron beam into the parallel electron beam.